Steel sheet and producing method therefor

ABSTRACT

A steel sheet including a chemical composition in mass %: C: 0.14-0.60%, Si+Al≤3.00, P 0.030%, S≤0.0050%, N 0.015%, B≤0.0050%, C×Mn≤0.80, Mn+Ni+Cu+1.3Cr+4(Mo+W)≥0.80, 0.003≤Ti+Zr+Hf+V+Nb+Ta+Sc+Y≤0.20, Sn+As+Sb+Bi≤0.020, Mg: 0 to 0.005%, Ca: 0 to 0.005%, REM: 0 to 0.005%, with the balance: Fe and impurities, and satisfying Ms=546 ×exp(− 1.362  x C)−11 ×Si−30 ×Mn−18 ×Ni−20 ×Cu−12×Cr − 8 (Mo+W)≥200.

TECHNICAL FIELD

The present invention relates to a steel sheet and a method for producing the steel sheet.

BACKGROUND ART

From the viewpoint of reducing a weight of an automobile body and ensuring collision safety, the application of a high-strength steel sheet as a steel sheet for an automobile has been sought. Members for an automobile include reinforcing members such as a bumper or a door guard bar as well as skeleton members such as a pillar, a sill, and a member. A high-strength steel sheet applied to these members is required to have such a collision resistance that can ensure safety of passengers at the time of collision (e.g., Patent Documents 1 to 3). Here, the collision resistance refers to properties having high reaction force properties and enabling absorbing energy at the time of crash deformation without causing a brittle fracture even when a member significantly deforms at the time of the crash deformation.

As a steel sheet excellent in energy absorption properties, a DP steel sheet having a duplex micro-structure of ferrite and martensite (e.g., Patent Document 4) or a TRIP steel sheet (transformation induced plasticity steel sheet) having a steel micro-structure of ferrite and bainite as well as retained y (e.g., Patent Document 5) is used. Further, steel sheets and members having a steel micro-structure made mainly of martensite and having high yield stresses are disclosed (e.g., Patent Documents 6 to 8).

LIST OF PRIOR ART DOCUMENTS Patent Document

-   Patent Document 1: JP2009−185355A -   Patent Document 2: JP2011−111672A -   Patent Document 3: JP2012−251239A -   Patent Document 4: JP11−080878A -   Patent Document 5: JP11−080879A -   Patent Document 6: JP2010−174280A -   Patent Document 7: JP2013−117068A -   Patent Document 8: JP2015−175050A

Non Patent Document

-   Non-Patent Document 1: “Atlas for Bainitic Microstructures Vol. 1”,     1992, The Iron and Steel Institute of Japan, p. 4 -   Non-Patent Document 2: Tadashi Maki, “Tekko no soshiki seigyo (in     Japanese) (Microstructure control in steels)”, 2015, Uchida Rokakuho -   Non-Patent Document 3: Liu Xiao, et al., “Lattice-parameter     variation with carbon content of martensite. I. X-ray-diffraction     experimental study”, Physical Review B, 52 (1995), pp. 9970−9978

SUMMARY OF INVENTION Technical Problem

However, the DP steel sheet or the TRIP steel sheet described in Patent Document 4 or 5 provides a low yield stress and insufficient reaction force properties, and additionally, a crack occurs in some cases at the time of crash deformation from its end face formed by shearing punching, failing to obtain a predetermined amount of energy absorption.

In addition, although the steel sheets described in Patent Documents 6 to 8 having a steel micro-structure made mainly of martensite provide a high yield stress, when the steel sheet is formed into a member, brittle cracking occurs in some cases at the time of crash deformation at a stress concentration such as a punching end face or a portion at which the sheet is bent, failing to absorb collision energy sufficiently.

The present invention has an objective to provide a steel sheet that exerts good reaction force properties when an impact load is applied to a shaped component from the steel sheet, is unlikely to cause a crack from an end face of the component or a region of the component bent at the time of the impact, and has a yield stress of 1000 MPa or more, and to provide a method for producing the steel sheet.

Solution to Problem

The present inventors conducted intensive studies about a technique to solve the problems described above, and consequently came to obtain the following findings.

(a) By optimizing a crystal structure of martensite and further decreasing its block size to a certain value or less, it is possible to prevent or reduce the occurrence and the propagation of a crack from a stress concentration at the time of a fast and large deformation.

(b) By optimizing components and optimizing a martensitic transformation starting temperature Ms, it is possible to prevent or reduce the occurrence and the propagation of a crack from a stress concentration at the time of a fast deformation.

(c) By having a high yield stress in addition to preventing or reducing the occurrence of a crack, high reaction force properties and impact energy absorption ability can be obtained.

The present invention is made based on such findings and has a gist of the following steel sheet and the following method for producing the steel sheet.

-   -   (1) A steel sheet having a chemical composition consisting of,         in mass %:     -   C: 0.14 to 0.60%,     -   Si: more than 0% to less than 3.00%,     -   Al: more than 0% to less than 3.00%,     -   Mn: 5.00% or less,     -   P: 0.030% or less,     -   S: 0.0050% or less,     -   N: 0.015% or less,     -   B: 0 to 0.0050%,     -   Ni: 0 to 5.00%,     -   Cu: 0 to 5.00%,     -   Cr: 0 to 5.00%,     -   Mo: 0 to 1.00%,     -   W: 0 to 1.00%,     -   Ti: 0 to 0.20%,     -   Zr: 0 to 0.20%,     -   Hf: 0 to 0.20%,     -   V: 0 to 0.20%,     -   Nb: 0 to 0.20%,     -   Ta: 0 to 0.20%,     -   Sc: 0 to 0.20%,     -   Y: 0 to 0.20%,     -   Sn: 0 to 0.020%,     -   As: 0 to 0.020%,     -   Sb: 0 to 0.020%,     -   Bi: 0 to 0.020%,     -   Mg: 0 to 0.005%,     -   Ca: 0 to 0.005%, and     -   REM: 0 to 0.005%,     -   with the balance: Fe and impurities, and     -   satisfying following formulas (i) to (v), wherein     -   a value of Ms expressed by a following formula (vi) is 200 or         more,     -   a steel micro-structure contains, in volume %:     -   martensite: 85% or more, and     -   retained austenite: 15% or less,     -   with the balance: bainite,     -   an average block size of martensite and bainite: 3.0 pm or less,     -   an average axial ratio of martensite and bainite: 1.0004 to         1.0100, and     -   a yield stress is 1000 MPa or more:

Si+Al≤3.00  (i)

C×Mn 0.80  (ii)

Mn+Ni+Cu+1.3Cr+4(Mo+W)≥0.80  (iii)

0.003 ≤Ti+Zr+Hf+V+Nb+Ta+Sc+Y≤0.20  (iv)

Sn+As+Sb+Bi≤0.020  (v)

Ms=546 ×exp(−1.362 ×C)−11 ×Si−30 ×Mn−18 ×Ni−20 ×Cu−12×Cr −8(Mo+W)  (vi)

where symbols of elements represent contents (mass %) of the elements in the steel sheet, and in a case where an element is not contained, zero is assigned to its symbol.

(2) The steel sheet according to the above (1), wherein an average particle size of iron carbides included in the steel micro-structure is 0.005 to 0.20 pm.

(3) The steel sheet according to the above (1) or (2), wherein the steel sheet includes a plating layer on a surface of the steel sheet.

(4) A method for producing the steel sheet according to any one of the above (1) to (3), wherein

a cast piece having the chemical composition according to the above (1) is subjected to a hot-rolling step, a cold-rolling step, an annealing step, and a heat treatment step in this order,

in the hot-rolling step, the steel sheet is cooled to room temperature at an average cooling rate for a range from a rolling finish temperature to 650° C. set at 8° C./s or more,

in the annealing step, the steel sheet is held within a temperature range from an Ac₃ point to (Ac3 point+100)° C. for 3 to 90 s, and

an average cooling rate for a range from 700° C. to (Ms point - 50°) C is set at 10° C./s or more, and

in the heat treatment step,

in a case where the Ms point is 250° C. or more,

a holding time for a temperature range from (Ms point+50) to 250° C. is set at 100 to 10000 s, and

in a case where the Ms point is less than 250° C.,

a holding time for a temperature range from (Ms point+80) to 100° C. is set at 100 to 50000 s:

where the Ms point (° C.) and the Ac3 point (° C.) are expressed by following formulas, where symbols of elements represent contents (mass %) of the elements in the steel sheet, and in a case where an element is not contained, zero is assigned to its symbol.

Ms=546 ×exp(−1.362 ×C)−11 ×Si−30 ×Mn−18 ×Ni−20 ×Cu−12×Cr −8(Mo+W)  (vi)

Ac₃=910−203 ×C^(0.5)+44.7(Si+Al)−30 ×Mn+700 ×P−15.2 ×Ni−26 ×Cu −11 ×Cr+31.5 ×Mo  (vii)

(5) A method for producing the steel sheet according to any one of the above (1) to (3), wherein

a cast piece having the chemical composition according to the above (1) is subjected to a hot-rolling step, an annealing step, and a heat treatment step in this order,

in the hot-rolling step, the steel sheet is cooled to room temperature at an average cooling rate for a range from a rolling finish temperature to 650° C. set at 8° C./s or more,

in the annealing step, the steel sheet is held within a temperature range from an Ac₃ to (Ac₃+100°) C for 3 to 90 s, and

an average cooling rate for a range from 700° C. to (Ms - 50°) C is set at 10° C./s or more, and

in the heat treatment step,

in a case where the Ms point is 250° C. or more,

a holding time for a temperature range from (Ms+50) to 250° C. is set at 100 to 10000 s, and

in a case where the Ms point is less than 250° C.,

a holding time for a temperature range from (Ms+80) to 100° C. is set at 100 to 50000 s:

where the Ms point (° C.) and the Ac₃ point (° C.) are expressed by following formulas, where symbols of elements represent contents (mass %) of the elements in the steel sheet, and in a case where an element is not contained, zero is assigned to its symbol.

Ms=546 ×exp(−1.362 ×C)−11 ×Si−30 ×Mn−18 ×Ni−20 ×Cu−12×Cr −8(Mo+W)  (vi)

Ac₃=910−203 ×C⁰⁵+44.7(Si+Al)−30 ×Mn+700 ×P−15.2 ×Ni−26 ×Cu −11 ×Cr+31.5 ×Mo  (vii)

(6) A method for producing the steel sheet according to any one of the above (1) to (3), wherein

a cast piece having the chemical composition according to the above (1) is subjected to a hot-rolling step and a heat treatment step in this order,

in the hot-rolling step, a rolling finish temperature is set at a Ar3 point or more, and

an average cooling rate for a range from a rolling finish temperature to (Ms -50°) C is set at 10° C./s or more, and

in the heat treatment step,

in a case where the Ms point is 250° C. or more,

a holding time for a temperature range from (Ms+50) to 250° C. is set at 100 to 10000 s, and

in a case where the Ms point is less than 250° C.,

a holding time for a temperature range from (Ms+80) to 100° C. is set at 100 to 50000 s:

where the Ms point (° C.) and the Ar3 point (° C.) are expressed by following formulas, where symbols of elements represent contents (mass %) of the elements in the steel sheet, and in a case where an element is not contained, zero is assigned to its symbol.

Ms=546 ×exp(−1.362 x C)−11 ×Si−30 ×Mn−18 ×Ni−20 ×Cu−12×Cr −8(Mo+W)  (vi)

Ar₃=910−310 ×C+33 ×Si−80 ×Mn−55 ×Ni−20 ×Cu−15 ×Cr −80 ×Mo  (viii)

Advantageous Effect of Invention

According to the present invention, it is possible to obtain a high-strength steel sheet that exerts good reaction force properties when an impact load is applied to a shaped component from the steel sheet, is unlikely to cause a crack from an end face of the component or a region of the component bent at the time of the impact, and has a yield stress of 1000 MPa or more.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 FIG. 1 is a diagram for describing a shape of a test piece used for a collision test.

DESCRIPTION OF EMBODIMENT

Requirements of the present invention will be described below in detail.

(A) Chemical Composition

Reasons for limiting a content of each element are as follows. In the following description, a symbol “%” for each content means “mass %”.

C: 0.14 to 0.60%

C (carbon) is an element that has effects of improving strength and refining a block size. In order to maintain a yield stress of 1000 MPa, a content ofC is set at 0.14% or more. On the other hand, if the content of C is more than 0.60%, an Ms point decreases, and an average axial ratio to be described below tends to increase. As a result, at the time of crash deformation, a brittle fracture occurs at a stress concentration, decreasing impact energy absorption ability. The content of C is therefore set at 0.14 to 0.60%. The content ofC is preferably 0.15% or more, more preferably 0.18% or more, and is preferably 0.50% or less.

Si: more than 0% to less than 3.00% and Al: more than 0% to less than 3.00%, and

Si+Al≤3.00  (i)

Si (silicon) and Al (aluminum) are elements useful in deoxidizing steel and has, in the present invention, an effect of increasing an average axial ratio of martensite, an effect of preventing or reducing the formation of iron carbide, and an effect of decreasing a block size of martensite, thereby preventing or reducing cracking in a member at the time of crash deformation to improve energy absorption ability. In order to obtain an effect of the deoxidation, Si and Al are to be contained at more than 0% each. Si and Al are preferably contained at 0.01% or more each.

However, if their total content is more than 3.00%, a tendency of a brittle fracture to occur at the time of crash deformation increases, thereby decreasing impact energy absorption ability. The total content of Si and Al is therefore set at 3.00% or less. The total content is preferably 2.50% or less. A lower limit of the total content is not limited to a particular value, but in order to obtain the effect of decreasing the block size reliably, the total content is preferably 0.10% or more.

Mn: 5.00% or less

Mn (manganese) is an element that has effects of preventing or reducing the formation of ferrite and improving yield stress and is additionally useful in controlling the average axial ratio. However, if a content of Mn is more than 5.00%, the Ms point decreases, and the average axial ratio to be described below tends to increase. As a result, at the time of crash deformation, a brittle fracture occurs at a stress concentration, decreasing impact energy absorption ability. The content of Mn is therefore set at 5.00% or less. The content of Mn is preferably 4.00% or less, 3.00% or less, or 2.00% or less.

In order to obtain the effect reliably, Mn is preferably contained at 0.01% or more.

C×Mn 0.80  (ii)

The product of the contents of C and Mn is a parameter that correlates with a brittle fracture at a stress concentration at the time of crash deformation. If the value of C×Mn is more than 0.80, the brittle fracture tendency increases, and thus the value is set at 0.80 or less. This value is preferably 0.60 or less, more preferably 0.40 or less.

P: 0.030% or less

P (phosphorus) is an element that contributes to the improvement of strength. However, if a content of P is more than 0.030%, a grain boundary fracture tendency at the time of crash deformation increases, thereby decreasing impact energy absorption ability. The content of P is therefore set at 0.030% or less. From the viewpoint of resistance weldability, the content of P is preferably 0.020% or less. A lower limit of the content is not limited to a particular value, but reducing the content to less than 0.001% leads to an increase in a production cost, and thus 0.001% is practically the lower limit.

S: 0.0050% or less

S (sulfur) is an impurity element, and if a content of S is more than 0.0050%, a fracture occurs from a punched portion or a bent portion at the time of a crash. The content of S is therefore set at 0.0050% or less. The content of S is preferably 0.0040% or less or 0.0030% or less. A lower limit of the content is not limited to a particular value, but reducing the content to less than 0.0002% leads to an increase in a production cost, and thus 0.0002% is practically the lower limit.

N: 0.015% or less

N (nitrogen) is an element available for controlling the average axial ratio. However, if a content of N is more than 0.015%, a toughness of the steel sheet decreases, resulting in a tendency of cracking to occur from a stress concentration at the time of a crash. The content of N is therefore set at 0.015% or less. The content of N is preferably 0.010% or less or 0.005% or less. A lower limit of the content is not limited to a particular value, but reducing the content to less than 0.001% leads to an increase in a production cost, and thus 0.001% is practically the lower limit.

B: 0 to 0.0050%

B (boron) is an element that has an effect of increasing a hardenability of the steel sheet and therefore may be contained when necessary. However, if a content of B is more than 0.0050%, cracking may occur at the time of crash deformation. The content of B is therefore set at 0.0050% or less. The content of B is preferably 0.0040% or less or 0.0030% or less. A lower limit of the content of B is not limited to a particular value and may be 0%, but when obtaining the effect described above is intended, the content of B is preferably 0.0003% or more.

Ni: 0 to 5.00%, Cu: 0 to 5.00%, Cr: 0 to 5.00%, Mo: 0 to 1.00%, and W: 0 to 1.00%, and

Mn+Ni+Cu+1.3Cr+4(Mo+W)≥0.80  (iii)

As with Mn, Ni (nickel), Cu (copper), Cr (chromium), Mo (molybdenum), and W (tungsten) are elements that have effects of preventing or reducing the formation of ferrite and improving yield stress and are additionally useful in controlling the average axial ratio. Thus, one or more elements selected from these elements may be contained. In order to obtain this effect, contents of these elements need to satisfy Formula (iii).

From the viewpoint of stably preventing or reducing the formation of ferrite and bainite, the left side value of Formula (iii) described above is preferably 1.00 or more. An upper limit of the left side value is not limited to a particular value, but if the left side value is more than 4.00, the Ms point decreases, and the average axial ratio to be described below tends to increase. As a result, at the time of crash deformation, a brittle fracture may occur at a stress concentration, decreasing impact energy absorption ability. The left side value of Formula (iii) described above is preferably 4.00 or less.

In addition, the contents of Ni and Cu are each preferably 4.00% or less, more preferably 3.00% or less, still more preferably 1.00% or less. The content of Cr is preferably 3.00% or less, more preferably 1.00% or less. The contents of Mo and Ware each preferably 0.80% or less, more preferably 0.60% or less.

Ti: 0 to 0.20%, Zr: 0 to 0.20%, Hf: 0 to 0.20%, V: 0 to 0.20%, Nb: 0 to 0.20%, Ta: 0 to 0.20%, Sc: 0 to 0.20%, and Y: 0 to 0.20%, and

0.003 Ti+Zr+Hf+V+Nb+Ta+Sc+Y≤0.20  (iv)

These elements have an effect of decreasing a block size of martensite and an effect of preventing or reducing the formation of iron carbide, thereby preventing or reducing the occurrence and the propagation of a crack from a stress concentration at the time of crash deformation. Thus, at least one or more of these elements are contained, and their total content is set at 0.003% or more. On the other hand, if the total content is more than 0.20%, alloy precipitate precipitates in a large quantity, and thus cracking tends to occur at the time of crash deformation; therefore, the total content is set at 0.20% or less. The total content is preferably 0.010% or more.

Sn: 0 to 0.020%, As: 0 to 0.020%, Sb: 0 to 0.020%, and Bi: 0 to 0.020%, and

Sn+As+Sb+Bi≤0.020  (v)

Sn (tin), As (arsenic), Sb (antimony), and Bi (bismuth) are elements each of which is used for obtaining a predetermined steel micro-structure, and therefore one or more elements selected from these elements may be contained when necessary. However, if their total content is more than 0.020%, a grain boundary fracture tendency at the time of crash deformation increases; therefore, an upper limit of the total content is set at 0.020%. A lower limit of the total content is not limited to a particular value, but reducing the total content to less than 0.00005% leads to an increase in a production cost, and thus 0.00005% is practically the lower limit.

Mg: 0 to 0.005%, Ca: 0 to 0.005%, and REM: 0 to 0.005%

Mg (magnesium), Ca (calcium), and REM (rare earth metal) are elements each of which has an action that controls morphology of oxides and sulfides, and therefore one or more elements selected from these elements may be contained when necessary.

However, if a content of any one of the elements is more than 0.005%, the effect provided by the addition of the element levels off, and energy absorption ability at the time of crash deformation decreases; therefore, the content of any one of the elements is set at 0.005% or less. Any one of the contents of Mg, Ca, and REM is preferably 0.003% or less. When obtaining the effect described above is intended, one or more elements selected from Mg: 0.001% or more, Ca: 0.001% or more, and REM: 0.001% or more are preferably contained.

Here, in the present invention, REM refers lanthanoids, which are 15 elements, and the content of REM means a total content of the lanthanoids. In industrial practice, the lanthanoids are added in a form of misch metal.

Value of Ms: 200 or more

Ms means a martensitic transformation starting temperature (° C.). If a Ms point of a steel sheet is less than 200° C., an axial ratio increases, and it becomes difficult for the configuration according to the present invention to prevent or reduce brittle fracture at the time of crash deformation. For that reason, a value of Ms is set at 200 or more. The value of Ms is preferably 220 or more.

Ms=546 ×exp(−1.362 ×C)−11 ×Si−30 ×Mn−18 ×Ni−20 ×Cu−12×Cr −8(Mo+W)  (vi)

In the chemical composition of the steel sheet according to the present invention, the balance is Fe and impurities. The term “impurities” as used herein means components that are mixed in the steel sheet in producing the steel sheet industrially due to raw materials such as ores and scraps, and various factors of a producing process and that are allowed to be mixed in the steel sheet within ranges in which the impurities have no adverse effect on the present invention.

(B) Steel Micro-Structure

A steel micro-structure of a steel sheet according to an embodiment of the present embodiment will be described. In the following description, the symbol “%” means “volume %”.

Martensite: 85% or more

Making the steel micro-structure mainly of martensite is indispensable for ensuring a yield stress of 1000 MPa or more. If a volume ratio of martensite is less than 85%, it becomes difficult to ensure the yield stress: 1000 MPa or more. For that reason, the volume ratio of martensite is set at 85% or more. In order to ensure the yield stress stably, the volume ratio of martensite is preferably 90% or more. Note that the martensite should be construed as including tempered martensite, that is, a martensite with carbides formed therein. In addition, the morphology of martensite may be any one of lath, butterfly, twin, lamella, and the like.

Retained austenite: 15% or less

Retained austenite is a steel micro-structure that is useful in improving formability and improving impact energy absorption properties. However, if its volume ratio is more than 15%, there are a tendency of yield stress to decrease and a tendency of brittle cracking to occur at the time of crash deformation. For that reason, the volume ratio of retained austenite is set at 15% or less. The volume ratio of retained austenite is preferably 12% or less. A lower limit of the volume ratio is not limited to a particular value, but the volume ratio is preferably 0.1% or more.

The remainder of the steel micro-structure is bainite. Here, the bainite includes lower bainite and upper bainite, and additionally, bainitic ferrite (α° B) described in Non Patent Document 1 is categorized as the bainite. Note that tempered martensite is difficult to subject to structure separation from bainite in some cases even with Reference Document 1. In such a case where the structure separation is difficult, the bainite is considered as martensite to calculate a structure separation fraction. Although there is no need to place an upper limit on an area fraction of the bainite being the balance, the area fraction is practically 15% or less, preferably 10% or less.

A volume ratio of a steel micro-structure is determined according to the following procedure. First, a 1/4 thickness portion of a surface of each steel sheet parallel to a rolling direction and a thickness direction of the steel sheet is mirror-polished and subjected to Nital etching. The surface is then subjected to structure observation under a scanning electron microscope (SEM) or further a transmission electron microscope (TEM), using a photograph of a steel micro-structure obtained by capturing the steel micro-structure, the point counting method or image analysis is performed to determine area fractions of martensite and bainite, and the area fractions are used as volume ratios. In addition, the volume ratio of the retained austenite is determined by the X-ray diffraction method. An area of a region to be observed is set at 1000 μm² or more when a SEM is used, or 100 μm² or more when a TEM is used.

Further, in the present invention, an average block size and an average axial ratio of martensite and bainite are also defined as follows.

Average block size of martensite and bainite: 3.0 μm or less

A block size of martensite influences the occurrence and the propagation of a brittle fracture at the time of crash deformation; the smaller a value of the block size is, the better impact properties are obtained. If the average block size is more than 3.0 μm, a fracture of the sheet may occur at a bent portion at the time of crash deformation; therefore, the average block size is set at 3.0 μm or less. The average block size is preferably 2.7 μm or less, 2.5 μm or less, or 2.4 μm or less.

Here, the block size will be described. As shown in a table in p. 223 of Non-Patent Document 2, martensite and bainite can be classified as being made up of 24 different crystal units (variants) as their substructures. One of methods for grouping these 24 variants is a method using Bain groups, which are described in p. 223 of Non-Patent Document 2, by which martensite and bainite can be classified into three crystal units. The block size in the present invention indicates an average size of group grains when the classification is performed using these Bain groups.

The average block size is measured according to the following procedure.

First, each steel sheet is cut such that its surface parallel to its rolling direction and its thickness direction serves as an observation surface, and the cross section is measured between a 1/4 sheet-thickness position and a 1/2 sheet-thickness position of the cross section by the EBSD method within a region having an area of 5000 μm² or more. A step size of the measurement is set at 0.2 μm. Then, based on crystal orientation information obtained by the EBSD measurement, orientations are classified on the basis of the three Bain groups, their images are displayed, and a size of a crystal unit is determined by the cutting method described in Appendix 2 of JIS G 0552.

Average axial ratio of martensite and bainite: 1.0004 to 1.0100

A crystal structure of a portion of the steel micro-structure other than the retained austenite, that is, martensite and bainite, influences cracking behavior at a stress concentration and a bent portion at the time of crash deformation. It is particularly necessary to appropriately adjust an average axial ratio of martensite and bainite, which have a tetragonal crystal structure. Here, the axial ratio is a value expressed by c/a, where a and c denotes a-axis and c-axis lattice constants in a tetragonal crystal structure, respectively. The reason that a magnitude of the axial ratio c/a is associated with cracking behavior at the time of a fast and large deformation in a collision test is unclear, but crystal lattice strain may exert some influence on the cracking behavior.

If the average axial ratio is less than 1.0004, cracking may occur at the time of crash deformation, or there arises a tendency to resist absorption of impact energy. On the other hand, if the average axial ratio is more than 1.0100, there arises a tendency of a brittle fracture to occur from an end face or a bent portion of a member at the time of crash deformation. For that reason, the average axial ratio is set at 1.0004 to 1.0100. From the viewpoint of ensuring the yield stress stably, the average axial ratio is preferably 1.0006 or more. Further, in order to prevent or reduce cracking at the time of crash deformation more reliably, the average axial ratio is preferably 1.0007 or more. On the other hand, from the viewpoint of increasing the absorption of impact energy, the average axial ratio is preferably 1.0080 or less.

Here, the average axial ratio of martensite and bainite is measured by the X-ray diffraction method according to the following procedure. At this time, the average axial ratio c/a is to be determined by any one of the following two methods depending on whether diffraction lines of tetragonal iron or cubic iron are split. Here, an area of a region on a sample irradiated with X-ray is set at 0.2 mm2 or more.

(a) In a case where a 200 diffraction line and a 002 diffraction line are split clearly into two

The pseudo-Voigt function is used to perform peak separation of diffraction lines from a {200} plane, a lattice constant calculated from a 200 diffraction angle is denoted by a, a lattice constant calculated from a 002 diffraction angle is denoted by c, and their ratio is determined as the average axial ratio c/a.

(b) In a case where the diffraction lines are not split clearly into two

A lattice constant calculated from a diffraction angle of a diffraction from a {200} plane is denoted by a, a lattice constant calculated from a diffraction angle from a {110} plane is denoted by c′, and their ratio c′/a is approximated as the average axial ratio c/a (see Non Patent Document 3).

Average particle size of iron carbides: 0.005 to 0.20 μm

Iron carbide may be contained in a steel micro-structure of a steel sheet according to another embodiment of the present invention. If an average particle size of iron carbides is more than 0.20 μm, a fracture from a bent portion tends to accelerate during crash deformation; on the other hand, if the average particle size of iron carbides is less than 0.005 μm, a brittle fracture from a bent portion during crash deformation tends to accelerate. For that reason, the average particle size of iron carbides is preferably 0.005 to 0.20 gm. Note that the iron carbide may contain, in addition to Fe, alloying elements such as Mn and Cr.

An average particle size of iron carbides in martensite and bainite is measured by observing their structures under a SEM and a TEM in a region having an area of 10 μm² or more. Fine iron carbides that cannot be identified with the TEM are measured by the atom probe method. In this case, the measurement is to be performed on five or more iron carbides.

(C) Plating layer

A steel sheet according to another embodiment of the present invention may include a plating layer on its surface. A composition of the plating is not limited to a particular composition, and any one of hot-dip galvanizing, galvannealing, and electroplating may be employed.

(D) Mechanical properties

Yield stress: 1000 MPa or more

If the yield stress is less than 1000 MPa, an advantage of reducing a member weight provided by making the member thin-wall, and the yield stress is therefore set at 1000 MPa or more. Here, the yield stress is determined to be a flow stress (0.2% proof stress) at 0.002 strain when a tensile test is performed in conformance with JIS Z 2241 2011.

Although there is no particular limitation imposed on a tensile strength, the tensile strength is preferably 1400 MPa or more from the viewpoint of enhancing impact energy absorption properties.

(E) Producing method

Although there is no particular limitation on conditions for producing the steel sheet according to the present invention, the steel sheet can be produced by subjecting a cast piece having the chemical composition described above to processing including steps described below as (a) to (c). Each of the methods will be described in detail.

Note that the cast piece can be obtained by a conventional method from a molten steel having the chemical composition described above. The cast piece to be subjected to hot rolling is not limited to a particular cast piece. That is, the cast piece may be a continuously cast slab or a cast piece produced by a thin slab caster. In addition, the method is applicable to a process such as continuous-casting direct-rolling, in which hot rolling is performed immediately after casting.

In the following description, the Ms point (° C.), the Ac3 point (° C.), and the Ar3 point (° C.) are expressed by the following formulas, where symbols of elements represent contents (mass %) of the elements in the steel sheet, and in a case where an element is not contained, zero is assigned to its symbol.

Ms=546 ×exp(−1.362 ×C)−11 ×Si−30 ×Mn−18 ×Ni−20 ×Cu−12×Cr −8(Mo+W)  (vi)

Ac₃=910−203 ×C⁰⁵+44.7(Si+Al)−30 ×Mn+700 ×P−15.2 ×Ni−26 ×Cu −11 ×Cr+31.5 ×Mo  (vii)

Ar₃=910−310 ×C+33 ×Si−80 ×Mn−55 ×Ni−20 ×Cu−15 ×Cr −80 ×Mo  (viii)

(a) Method including hot-rolling step, cold-rolling step, annealing step, and heat treatment step

The cast piece described above is subjected to a hot-rolling step, a cold-rolling step, an annealing step, and a heat treatment step in this order. In this case, a resultant steel sheet is a cold-rolled steel sheet. Each of the steps will be described in detail.

In the hot-rolling step, the cast piece is first heated. The heating temperature is not limited to a particular temperature but is preferably set at 1200° C. or more so that alloy carbo-nitride that has precipitated during casting or rough rolling is remelted.

After the heating, hot rolling is performed. At this time, an average cooling rate for the range from a rolling finish temperature to 650° C. is set at 8° C./s or more. If the average cooling rate is less than 8° C./s, a block size of martensite in a finished product increases, resulting in a deterioration in impact properties. Thereafter, the steel sheet is coiled. The coiling temperature is not limited to a particular temperature but is preferably 630° C. or less. After being coiled, the steel sheet is further cooled to room temperature.

Subsequently, the steel sheet is subjected to treatment such as pickling then cold rolling. As conditions for the cold rolling, the number of rolling passes and a rolling reduction need not be particularly specified, and the conditions are only required to conform to the conventional method.

In the annealing step, the steel sheet subjected to the cold rolling is retained within the temperature range from the Ac₃ point to (Ac₃ point+100°) C for 3 to 90 s. If the annealing temperature is less than the Ac₃ point, a predetermined amount of martensite cannot be obtained, and if the annealing temperature is more than (Ac₃ point+100°) C, block size increases. In addition, if the retention time for the temperature range is less than 3 s, the predetermined amount of martensite is not obtained, and a yield stress of 1000 MPa or more cannot be obtained. On the other hand, if the retention time is more than 90 s, the block size increases. From the viewpoint of decreasing the block size, the annealing temperature is preferably as low as possible and is preferably (Ac₃ point+80°) C or less. In addition, the retention time is preferably 10 s or more and is preferably 60 s or less.

After being retained within the temperature range for a predetermined time period, the steel sheet is cooled under a condition that an average cooling rate for the range from 700° C. to (Ms point - 50°) C is 10° C./s or more. If this average cooling rate is less than 10° C./s, the predetermined amount of martensite cannot be obtained, resulting in the yield stress to decrease, and further, the block size increases, resulting in a tendency of cracking to occur at the time of impact deformation. In a case where the setting of the average axial ratio at 1.0007 or more is intended for preventing or reducing cracking at the time of crash deformation more reliably, the average cooling rate is preferably 20° C./s or more. Note that a temperature at which this cooling is stopped is only required to be (Ms - 50°) C or less and is not limited to a particular temperature but is preferably 100° C. or more from the viewpoint of resistance to fracture.

In the heat treatment step, a heat treatment that results in the following thermal history is performed, according to Ms calculated from the chemical composition of the steel sheet. Note that the following heat treatment may be performed subsequently to stopping the cooling, or heating may be performed to a degree that does not exceed an upper limit of a temperature range in the heat treatment step described below subsequently to stopping the cooling.

In a case where the Ms point calculated from the chemical composition of the steel sheet is 250° C. or more, a holding time for the temperature range from (Ms point+50) to 250° C. is set at 100 to 10000 s. If the holding time is less than 100 s, the average axial ratio may exceed a predetermined value, causing a brittle fracture in a collision test or failing to obtain a predetermined yield stress. On the other hand, if the holding time is more than 10000 s, the average axial ratio becomes less than a predetermined value, and additionally iron carbides coarsen, resulting in a tendency of cracking to occur at the time of a crash. The holding time is preferably 400 s or more and is preferably 5000 s or less. In particular, in a case where the setting of the average axial ratio at 1.0007 or more is intended for preventing or reducing cracking at the time of crash deformation more reliably, the holding time is more preferably 1500 s or less.

In a case where the Ms point calculated from the chemical composition of the steel sheet is less than 250° C., a holding time for the temperature range from (Ms point+80) to 100° C. is set at 100 to 50000 s. If the holding time is less than 100 s, the average axial ratio may exceed the predetermined value, causing a brittle fracture in a collision test. On the other hand, if the holding time is more than 50000 s, the average axial ratio becomes less than a predetermined value, and additionally iron carbides coarsen, resulting in a tendency of cracking to occur at the time of a crash. The holding time is preferably 400 s or more and is preferably 30000 s or less, more preferably 10000 s or less.

(b) Method including hot-rolling step, annealing step, and heat treatment step

The cast piece described above is subjected to a hot-rolling step, an annealing step, and a heat treatment step in this order. In this case, a resultant steel sheet is a hot-rolled steel sheet. Each of the steps will be described in detail.

In contrast to the steps described in (a), the cold-rolling step is not performed in the present step. In the annealing step, ferrite being a parent phase is recrystallized while the cold-rolled steel sheet is heated from room temperature to the annealing temperature, and crystallographic texture develops. Under the influence of this preferential orientation of the crystal orientations, crystallographic texture also develops in austenite that exists in retention within the temperature range from the Ac₃ point to (Ac₃ point+100°) C. By the development of the crystallographic texture, when austenite with a biased orientation transforms to martensite, crystals of the martensite are formed and grow in a particular direction.

In addition, since the formation and the growth of crystals of the martensite causes the steel to expand, the steel sheet expands biasedly in the particular direction macroscopically. However, allowing a steel strip to expand or deform freely in the annealing step leads to a decrease in strip running properties; therefore, a tension is usually applied to straighten a shape of the steel sheet and to keep the stability of strip running.

Note that if martensitic transformation occurs in a state where such an excessive tension is applied, a residual stress is applied in the steel sheet, and it becomes difficult to obtain an effect of preventing or reducing cracking. In addition, if the residual stress in the steel sheet increases, a crack that occurs when the steel sheet is deformed is likely to form and propagate. For that reason, from the viewpoint of preventing or reducing cracking at the time of crash deformation more reliably, the cold-rolling step is preferably omitted; that is, with an aim of preventing the development of crystallographic texture by the recrystallization of ferrite being a parent phase in the annealing step to align the crystal orientations on a random basis, the steel sheet according to an embodiment of the present invention is preferably a hot-rolled steel sheet.

In the hot-rolling step, the cast piece is first heated. The heating temperature is not limited to a particular temperature but is preferably set at 1200° C. or more so that alloy carbo-nitride that has precipitated during casting or rough rolling is remelted.

After the heating, hot rolling is performed. At this time, an average cooling rate for the range from a rolling finish temperature to 650° C. is set at 8° C./s or more. If the average cooling rate is less than 8° C./s, a block size of martensite in a finished product increases, resulting in a deterioration in impact properties. Thereafter, the steel sheet may be coiled or need not be coiled but may be cooled to room temperature. In addition, after the cooling, the steel sheet may be subjected to treatment such as pickling or may be subjected to flattening.

In the annealing step, the steel sheet subjected to the hot rolling is retained within the temperature range from the Ac₃ point to (Ac₃ point+100°) C for 3 to 90 s. If the annealing temperature is less than the Ac₃ point, a predetermined amount of martensite cannot be obtained, and if the annealing temperature is more than (Ac₃ point+100°) C, block size increases. In addition, if the retention time for the temperature range is less than 3 s, the predetermined amount of martensite is not obtained, and as a result, a yield stress of 1000 MPa or more cannot be obtained. On the other hand, if the retention time is more than 90 s, the block size increases. From the viewpoint of decreasing the block size, the annealing temperature is preferably as low as possible and is preferably (Ac₃ point+80°) C or less. In addition, the retention time is preferably 10 s or more and is preferably 60 s or less.

After being retained within the temperature range for a predetermined time period, the steel sheet is cooled under a condition that an average cooling rate for the range from 700° C. to (Ms point - 50°) C is 10° C./s or more. If this average cooling rate is less than 10° C./s, the predetermined amount of martensite cannot be obtained, resulting in the yield stress to decrease, and further, the block size increases, resulting in a tendency of cracking to occur at the time of impact deformation. In a case where the setting of the average axial ratio at 1.0007 or more is intended for preventing or reducing cracking at the time of crash deformation more reliably, the average cooling rate is preferably 20° C./s or more. Note that a temperature at which this cooling is stopped is only required to be (Ms - 50°) C or less and is not limited to a particular temperature but is preferably 100° C. or more from the viewpoint of resistance to fracture.

In the heat treatment step, a treatment that results in the following thermal history is performed, according to Ms calculated from the chemical composition of the steel sheet. Note that the following heat treatment may be performed subsequently to stopping the cooling in the annealing step, or heating may be performed to a degree that does not exceed an upper limit of a temperature range in the heat treatment step described below subsequently to stopping the cooling.

In a case where the Ms point calculated from the chemical composition of the steel sheet is 250° C. or more, a holding time for the temperature range from (Ms point+50) to 250° C. is set at 100 to 10000 s. If the holding time is less than 100 s, the average axial ratio may exceed a predetermined value, causing a brittle fracture in a collision test or failing to obtain a predetermined yield stress. On the other hand, if the holding time is more than 10000 s, the average axial ratio becomes less than a predetermined value, and additionally iron carbides coarsen, resulting in a tendency of cracking to occur at the time of a crash. The holding time is preferably 400 s or more and is preferably 5000 s or less. In particular, in a case where setting of the average axial ratio at 1.0007 or more is intended for preventing or reducing cracking at the time of crash deformation more reliably, the holding time is more preferably 1500 s or less.

In a case where the Ms point calculated from the chemical composition of the steel sheet is less than 250° C., a holding time for the temperature range from (Ms point+80) to 100° C. is set at 100 to 50000 s. If the holding time is less than 100 s, the average axial ratio may exceed the predetermined value, causing a brittle fracture in a collision test. On the other hand, if the holding time is more than 50000 s, the average axial ratio becomes less than a predetermined value, and additionally iron carbides coarsen, resulting in a tendency of cracking to occur at the time of a crash. The holding time is preferably 400 s or more and is preferably 30000 s or less, more preferably 10000 s or less.

(c) Method including hot-rolling step and heat treatment step

The cast piece described above is subjected to a hot-rolling step and a heat treatment step in this order. In this case, a resultant steel sheet is a hot-rolled steel sheet. Each of the steps will be described in detail.

In contrast to the steps described in (b), the annealing step is not performed in the present step. When the annealing is performed, while the heating is performed from the room temperature to the annealing temperature in the annealing step, boundary motion of a martensitic structure occurs. Further, because a crystal interface in a particular orientation of a high mobility preferentially moves in this boundary motion, the randomization of crystal orientations is lost, and a slight residual stress remains in the steel sheet subjected to the annealing step. For that reason, from the viewpoint of reducing the residual stress as much as possible, the annealing step is preferably omitted.

In the hot-rolling step, the cast piece is first heated. The heating temperature is not limited to a particular temperature but is preferably set at 1200° C. or more so that alloy carbo-nitride that has precipitated during casting or rough rolling is remelted.

After the heating, hot rolling is performed. At this time, the hot rolling is performed such that the rolling finish temperature becomes the Ar₃ point or more. If the rolling finish temperature is less than the Ar₃ point, ferrite is formed, which makes it difficult to obtain the predetermined yield stress.

After the hot rolling, the steel sheet is cooled under a condition that an average cooling rate for the range from the rolling finish temperature to (Ms point - 50°) C is 10° C./s or more. If this average cooling rate is less than 10° C./s, a volume ratio of ferrite or bainite increases, the predetermined amount of martensite cannot be obtained, resulting in the yield stress to decrease, and further, the block size increases, resulting in a tendency of cracking to occur at the time of impact deformation. Note that a temperature at which this cooling is stopped is only required to be (Ms - 50°) C or less and is not limited to a particular temperature but is preferably 100° C. or more from the viewpoint of resistance to fracture.

In the heat treatment step, a treatment that results in the following thermal history is performed, according to Ms calculated from the chemical composition of the steel sheet. Note that the following heat treatment may be performed subsequently to stopping the cooling in the hot-rolling step, or heating may be performed to a degree that does not exceed an upper limit of a temperature range in the heat treatment step described below subsequently to stopping the cooling.

In a case where the Ms point calculated from the chemical composition of the steel sheet is 250° C. or more, a holding time for the temperature range from (Ms point+50) to 250° C. is set at 100 to 10000 s. If the holding time is less than 100 s, the average axial ratio may exceed a predetermined value, causing a brittle fracture in a collision test or failing to obtain a predetermined yield stress. On the other hand, if the holding time is more than 10000 s, the average axial ratio becomes less than a predetermined value, and additionally iron carbides coarsen, resulting in a tendency of cracking to occur at the time of a crash. The holding time is preferably 400 s or more and is preferably 5000 s or less. In particular, in a case where the setting of the average axial ratio at 1.0007 or more is intended for preventing or reducing cracking at the time of crash deformation more reliably, the holding time is more preferably 1500 s or less.

In a case where the Ms point calculated from the chemical composition of the steel sheet is less than 250° C., a holding time for the temperature range from (Ms point+80) to 100° C. is set at 100 to 50000 s. If the holding time is less than 100 s, the average axial ratio may exceed the predetermined value, causing a brittle fracture in a collision test. On the other hand, if the holding time is more than 50000 s, the average axial ratio becomes less than a predetermined value, and additionally iron carbides coarsen, resulting in a tendency of cracking to occur at the time of a crash. The holding time is preferably 1000 s or more and is preferably 30000 s or less, more preferably 10000 s or less.

After any one of the steps of (a) to (c) is finished, skin-pass rolling may be performed for flattening. The elongation percentage is not limited to a particular percentage. In addition, plating treatment may be performed in the middle of the heat treatment or after the heat treatment is finished, as far as the thermal history is satisfied.

As a method of plating, the steel sheet may be produced in a continuous-annealing and plating line or may be produced by using a plating-dedicated facility separate from an annealing line. A composition of the plating is not limited to a particular composition, and any one of hot-dip galvanizing, galvannealing, and electroplating may be employed.

The present invention will be described below more specifically with reference to examples, but the present invention is not limited to these examples.

Example 1

Steels having compositions shown in Table 1 were melted and produced into slabs, and the slabs were heated at 1220 to 1260° C. and subjected to rough rolling performed as a hot processing. Subsequently, steel sheets were subjected to finish rolling, cooled, then subjected to coiling processing at 500 to 620° C., and cooled to room temperature. Then, as shown in Tables 2 and 3, the average cooling rate (CR1) for the range from the rolling finish temperature (FT) to 650° C. was changed.

After being cooled to the room temperature, the steel sheets were subjected to pickling treatment for removing scales, then subjected to cold rolling at a cold rolling ratio of 30 to 70% so that the steel sheets had a thickness of 1.2 mm, and then annealed.

In the annealing, the annealing temperature (ST), the annealing retention time (t1), and the average cooling rate (CR2) for the range from 700° C. to (Ms point - 50°) C were changed, and in the heat treatment step, the holding time (t2) for the range from (Ms +50°) C to 250° C. was changed for steels having Ms being 250° C. or more, and the holding time (t3) for the range from (Ms+80°) C to 100° C. was changed for steels having Ms being less than 250° C. After the heat treatment step, skin-pass rolling for flattening was performed.

TABLE 1 Chemical composition (mass %, balance: Fe and impurities) Steel C Si Al Mn P S N B Ni Cu Cr Mo W A 0.19 0.10 0.05 2.00 0.015 0.0010 0.002 0.0010 0.50 — 0.30 — — B 0.19 0.30 0.05 1.50 0.015 0.0010 0.002 0.0010 — — — 0.50 — C 0.30 0.30 0.05 1.30 0.015 0.0010 0.002 0.0010 0.60 0.10 0.10 — 0.40 D 0.30 1.50 0.05 1.90 0.015 0.0010 0.002 0.0004 0.50 — — 0.20 — E 0.30 0.70 1.10 1.60 0.015 0.0010 0.003 0.0004 0.30 0.20 0.30 — — F 0.30 0.70 1.10 1.30 0.015 0.0010 0.003 0.0004 0.80 0.20 0.20 — — G 0.30 1.70 0.05 2.40 0.015 0.0010 0.002 0.0004 0.10 — 0.20 0.10 — H 0.40 0.10 0.05 1.00 0.015 0.0010 0.002 0.0010 — — 0.60 0.30 — I 0.48 0.10 0.05 1.00 0.015 0.0010 0.002 0.0010 0.70 — 0.20 — 0.30 J 0.14 0.30 0.05 1.80 0.015 0.0010 0.002 0.0010 — — — 0.10 — K 0.65 0.30 0.05 1.50 0.015 0.0010 0.002 0.0010 — — — 0.50 — L 0.12 0.30 0.05 1.50 0.015 0.0010 0.002 0.0010 — — — 0.50 — M 0.30 3.50 0.05 1.50 0.015 0.0010 0.002 0.0010 — — — 0.50 — N 0.30 1.70 1.50 1.50 0.015 0.0010 0.002 0.0010 — — — 0.50 — O 0.30 0.02 0.02 1.50 0.015 0.0010 0.002 0.0010 — — — 0.50 — P 0.30 0.30 0.05 1.50 0.050 0.0010 0.002 0.0010 — — — 0.50 — Q 0.30 0.30 0.05 1.50 0.015 0.0100 0.002 0.0010 — — — 0.50 — R 0.30 0.30 0.05 1.50 0.015 0.0010 0.002 0.0200 — — — 0.50 — S 0.30 0.30 0.05 3.00 0.015 0.0010 0.002 0.0010 — — — — — T 0.30 0.30 0.05 0.01 0.015 0.0010 0.002 — 3.00 — — — — U 0.30 0.30 0.05 0.60 0.015 0.0010 0.002 0.0010 — — — — — V 0.48 0.30 0.05 1.00 0.015 0.0010 0.002 0.0010 0.80 0.80 — — — W 0.48 0.30 0.05 1.50 0.015 0.0010 0.002 0.0010 2.10 — — — — X 0.22 0.30 0.05 4.00 0.015 0.0010 0.002 0.0010 — — — — — Y 0.30 0.30 0.05 1.50 0.015 0.0008 0.002 0.0010 — — — — — Z 0.30 0.30 0.05 1.50 0.015 0.0008 0.002 0.0010 — — — — — AA 0.30 0.30 0.05 1.50 0.015 0.0008 0.002 0.0010 — — — — — Chemical composition (mass %, balance: Left side Left side Left side Middle side Left side M_(s) Ac₃ Ar₃ Fe and impurities) value of vale of value of value of value of Formula Formula Formula Steel Others Formula (i) Formula (ii) Formula (iii) Formula (iv) Formula (v) (vi) (vii) (viii) A Ti: 0.02, Nb: 0.02, 0.15 0.38 2.89 0.04 0 348 768 662 REM: 0.002 B Ti: 0.02, Nb: 0.02 0.35 0.29 3.50 0.04 0 369 818 701 C Ti: 0.03, Zr: 0.01, 0.35 0.39 3.73 0.05 0 303 773 686 Hb: 0.01, Mg: 0.002 D Ti: 0.02, V: 0.15, 1.55 0.57 3.20 0.18 0 279 820 671 Ta: 0.01, Ca: 0.001 E Ti: 0.03, Zr: 0.001, 1.80 0.48 2.49 0.03 0.011 294 829 687 REM: 0.001, Sb: 0.010, Bi: 0.001 F Zr: 0.01, Nb: 0.01, 1.80 0.39 2.56 0.02 0.011 295 831 685 Sb: 0.010, Bi: 0.001 G Ti: 0.03, Zr: 0.01, 1.75 0.72 3.16 0.05 0.016 267 815 665 Y: 0.01, Sb: 0.010, Sn: 0.003, As: 0.003, Mg: 0.001 H Nb: 0.05, V: 0.14, 0.15 0.40 2.98 0.19 0.001 276 772 676 REM: 0.001, Bi: 0.001 I Ti: 0.02, Sc: 0.04, 0.15 0.48 3.16 0.06 0 235 744 643 Mg: 0.002 J Ti: 0.02, Nb: 0.01 0.35 0.25 2.20 0.03 0 393 809 725 K Ti: 0.02, Nb: 0.01 0.35 0.98 3.50 0.03 0 173 743 558 L Ti: 0.02, Nb: 0.01 0.35 0.18 3.50 0.03 0 411 837 723 M Ti: 0.02, Nb: 0.01 3.55 0.45 3.50 0.03 0 275 930 773 N Ti: 0.02, Nb: 0.01 3.20 0.45 3.50 0.03 0 295 923 713 O Ti: 0.02, Nb: 0.01 0.04 0.45 3.50 0.03 0 314 782 658 P Ti: 0.02, Nb: 0.01 0.35 0.45 3.50 0.03 0 311 820 667 Q Ti: 0.02, Nb: 0.01 0.35 0.45 3.50 0.03 0 311 796 667 R Ti: 0.02, Nb: 0.01 0.35 0.45 3.50 0.03 0 311 796 667 S Ti: 0.02, Nb: 0.01 0.35 0.90 3.00 0.03 0 270 735 587 T Ti: 0.02, Nb: 0.01 0.35 0.00 3.01 0.03 0 305 779 661 U Ti: 0.02, Nb: 0.01 0.35 0.18 0.60 0.03 0 342 807 779 V Ti: 0.02, Nb: 0.01 0.35 0.48 2.60 0.03 0 220 733 631 W Ti: 0.02, Nb: 0.01 0.35 0.72 3.60 0.03 0 198 719 536 X Ti: 0.02, Nb: 0.01 0.35 0.88 4.00 0.03 0 281 721 532 Y — 0.35 0.45 1.50 0   0 315 780 707 Z Ti: 0.25 0.35 0.45 1.50 0.25 0 315 780 707 AA Ti: 0.03, Nb: 0.20 0.35 0.45 1.50 0.23 0 315 780 707 #¹ Si + Al ≤ 3.0 . . . (i) #² C × Mn ≤ 0.8 . . . (ii) #³ Mn + Ni + Cu + 1.3Cr + 4(Mo + W) ≥ 0.8 . . . (iii) #⁴ 0.003 ≤ Ti + Zr + Hf + V + Nb + Ta + Se + Y ≤ 0.2 . . . (iv) #⁵ Sn + As + Sb + Bi ≤ 0.02 . . . (v) #⁶ Ms = 546 × exp(−1.362 × C) − 11 × Si − 30 × Mn − 18 × Ni − 20 × Cu − 12 × Cr − 8(Mo + W) . . . (vi) #⁷ Ac₃ = 910 − 203 × C^(0.5) + 44.7(Si + Al) − 30 × Mn + 700 × P − 15.2 × Ni − 26 × Cu − 11 × Cr + 31.5 × Mo . . . (vii) #⁸ Ar₃ = 910 − 310 × C − 33 × Si − 80 × Mn − 55 × Ni − 20 × Cu − 15 × Cr − 80 × Mo . . . (viii)

TABLE 2 Aver- age Eval- uation Test No. Steel FT (° C.) CR1 (° C./s) ST (° C.) t1 (s) CR2 (° C./s) t2 (s) t3 (s) fM (%) fA (%) Bal- ance axial ratio db (μm) dear (μm) YS (MPa) TS (MPa) of cracking  1 A 900 10 830 30 60  400 99.8  0.2 1.0009 2.1 0.03 1100 1450 C Inventive example  2 900 10 868 30 60  400 99.8  0.2 1.0009 2.7 0.03 1100 1440 D Inventive example  3 900 10 910 30 60  400 99.8  0.2 1.0009 3.4 0.03 1100 1450 E Comparative example  4 900 10 790 30 60  400 99.8  0.2 1.0009 2.0 0.03 1120 1460 C Inventive example  5 900 10 750 30 00  400 83  0.2 F 1.0006 1.9 0.03  890 1450 D Comparative example  6 900 10 790  1 60  400 84  0.2 F 1.0004 2.0 0.15  920 1450 D Comparative example  7 900 10 790 30 15  400 96  0.2 B 1.0006 2.0 0.04 1000 1460 D Inventive example  8 900 10 790 30  5   400 80  0.2 B 1.0004 3.4 0.06  820 1430 E Comparative example  9 900 10 790 30 60     3 99.8  0.2 1.0051 2.0 0.003  980 1440 C Comparative example 10 900 10 790 30 60   900 99.8  0.2 1.0007 2.0 0.18 1130 1450 C Inventive example 11 900 10 790 30 60 20000 99.8  0.2 1.0002 2.0 0.22 1200 1440 E Comparative example 12 900  3 790 30 60  400 99.8  0.2 1.0009 3.1 0.04 1090 1450 D Comparative example 13 000 30 790 30 60  400 99.8  0.2 1.0009 2.0 0.04 1130 1460 C Inventive example 14 900 10 790 30 60  400 99.8  0.2 1.0009 2.2 0.05 1110 1440 C Inventive example 15 900 10 700 30 60  400 99.8  0.2 1.0009 1.5 0.05 1140 1470 C Inventive example 16 B 900 10 830 30 60  400 99.8  0.2 1.0012 2.1 0.07 1100 1450 C Inventive example 17 C 900 10 830 30 60  400 92  2 B 1.0015 2.0 0.11 1250 1700 C Inventive example 18 D 900 10 840 30 60  500 90 10 1.0015 1.8 0.12 1030 1500 C Inventive example 19 900 10 800 30 60  500 83 10 F 1.0015 1.8 0.12  800 1480 C Comparative example 20 900 10 930 30 60  500 92  8 1.0016 3.2 0.10 1040 1490 E Comparative example 21 900 10 840  1 60  500 84 10 F 1.0004 1.9 0.15  820 1290 D Comparative example 22 900 10 840 30  5  500 82 10 B 1.0004 3.8 0.21  900 1480 E Comparative example 23 900 10 840 30 15  5000 89 10 B 1.0004 2.4 0.18 1000 1490 D Inventive example 24 900 10 840 30 60     3 90 10 1.0110 1.8 0.002  950 1480 E Comparative example 25 900 10 840 30 60   200 90 10 1.0088 1.8 0.004 1010 1480 D Inventive example 26 900 10 840 30 60  1000 90  9 B 1.0011 1.8 0.12 1060 1490 C Investive example 27 900 10 S40 30 60 12000 90  6 B 1.0003 1.8 0.22 1090 1410 E Comparative example 28 900  5 840 30 60   500 90 10 1.0015 3.1 0.12 1050 1480 E Comparative example 29 E 900 10 840 30 60   500 85 11 B 1.0008 1.9 0.03 1020 1500 C Inventive example 30 F 900 10 840 30 60   500 85 12 B 1.0007 1.8 0.02 1030 1510 C Inventive example 31 G 900 10 840 30 60   500 89 11 1.0009 1.9 0.02 1150 1600 C Inventive example

TABLE 3 Aver- Eval- CR1 CR2 age uation Test FT (° C./ ST t1 (° C./ t2 t3 fM fA Bal- axial db dear YS TS of No. Steel (° C.) s) (° C.) (s) s) (s) (s) (%) (%) ance ratio (μm) (μm) (MPa) (MPa) cracking 32 H 900 10 840 30 60 900  97 3 1.0032 1.6 0.04 1300 1850 C Inventive example 33 I 900 10 800 30 60   600  96 4 1.0090 1.3 0.03 1450 2000 C Inventive example 34 900 10 720 30 60   600  83 5 F, B 1.0040 1.5 0.28  990 1950 C Comparative example 35 900 10 860 30 60   600  96 4 1.0091 3.5 0.02 1400 1980 E Comparative example 36 900 10 800  1 60   600  84 4 F 1.0022 1.5 0.15  980 1950 C Comparative example 37 900 10 800 30  5   600  82 5 B, F 1.0033 3.1 0.08  940 1970 E Comparative example 38 900 10 800 30 15   600  88 5 B 1.0033 1.7 0.16 1340 1990 C Inventive example 39 900 10 800 30 60      3  96 4 1.0230 1.6 0.001 1100 2020 E Comparative example 40 900 10 800 30 60     30  96 4 1.0200 1.6 0.003 1210 2010 E Comparative example 41 900 10 800 30 60    200  96 4 1.0000 1.6 0.015 1330 2010 C Inventive example 42 900 10 800 30 60   1000  96 4 1.0070 1.6 0.028 1400 2000 C Inventive example 43 900 10 800 30 60  12000  96 4 1.0010 1.6 0.028 1410 1980 C Inventive example 44 900 10 800 30 60 100000  96 4 1.0003 1.6 0.16 1550 1980 E Comparative example 45 J 900 10 820 30 60 300 99.8 0.2 1.0006 2.4 0.04 1020 1250 D Inventive example 46 K 900 10 830 30 60    600  93 7 1.0240 1.2 0.03 1500 2150 E Comparative example 47 L 900 10 850 30 60 300 100 0 1.0004 4.5 0.02  880 1150 E Comparative example 48 M 900 10 940 30 60 300  98 2 1.0007 1.3 0.008 1050 1550 E Comparative example 49 N 900 10 940 30 60 300  98 2 1.0007 1.2 0.005 1050 1500 E Comparative example 50 O 900 10 830 30 60 300  99 1 1.0007 2.8 0.01 1150 1650 D Inventive example 51 P 900 10 830 30 60 300  99 1 1.0007 1.8 0.01 1150 1650 E Comparative example 52 Q 900 10 830 30 60 300  99 1 1.0007 1.7 0.01 1150 1650 E Comparative example 53 R 900 10 830 30 60 300  99 1 1.0007 1.8 0.01 1200 1700 E Comparative example 54 S 900 10 830 30 60 300  99 1 1.0007 1.7 0.01 1220 1700 E Comparative example 55 T 900 10 830 30 60 300  97 3 1.0007 1.7 0.01 1220 1680 C Inventive example 56 U 900 10 830 30 60 300  80 3 F, B 1.0006 1.8 0.01  970 1450 D Comparative example 57 V 900 10 750 30 60   600  96 4 1.0079 1.4 0.01 1250 1710 C Inventive example 58 W 900 10 750 30 60   600  94 6 1.0220 1.3 0.01 1240 1720 E Comparative example 59 X 900 10 750 30 60 300  97 3 1.0008 2.8 0.01 1030 1400 E Comparative example 60 Y 900 10 820 30 60 300  99 1 1.0007 3.5 0.01 1150 1650 B Comparative example 61 Z 900 10 820 30 60 300  99 1 1.0008 1.4 0.004 1150 1650 E Comparative example 62 AA 900 10 820 30 60 300  99 1 1.0007 1.2 0.004 1140 1640 E Comparative example

Next, steel micro-structure observation was performed on the resultant steel sheets, and volume ratios of steel micro-structures were measured. Specifically, a 1/4 thickness portion of a surface of each steel sheet parallel to a rolling direction and a thickness direction of the steel sheet was mirror-polished, and the surface subjected to Nital etching was observed under a SEM. Using a photograph of its steel micro-structure, the measurement was performed by the point counting method to determine area fractions of steel micro-structures, and their values were used as the volume ratios of the steel micro-structures. At this time, an area of the observation was set at 2500 μm² or more. In addition, the volume ratio of retained austenite was measured by the X-ray diffraction method.

Note that, in the column “Remaining structure” of the tables, F indicates ferrite, B indicates bainite, and P indicates pearlite, and in the tables, fM and fA indicate the volume ratios of martensite and retained austenite with respect to all steel micro-structures, respectively.

The average block size of martensite and bainite was measured according to the following procedure. First, each steel sheet was cut such that its surface parallel to its rolling direction and its thickness direction served as an observation surface, and the cross section was measured between a 1/4 sheet-thickness position and a 1/2 sheet-thickness position of the cross section by the EBSD method within a region having an area of 5000 μm² or more. A step size of the measurement was set at 0.2 μm.

Next, based on crystal orientation information obtained by the EBSD measurement, orientations are classified on the basis of the three Bain groups, which are shown in the Table in p. 223 of Non-Patent Document 2. Next, with boundaries between these groups considered as block boundaries, and regions surrounded by these boundaries considered as block grains, sizes of the block grains (db) were determined by the cutting method described in Appendix 2 of JIS G0552.

The average axial ratio of martensite and bainite was measured by the X-ray diffraction method according to the following procedure. At this time, the axial ratio c/a was measured by any one of the following two methods depending on whether diffraction lines of tetragonal iron or cubic iron were split, and the average axial ratio was determined.

(a) In a case where a 200 diffraction line and a 002 diffraction line are split clearly into two

The pseudo-Voigt function was used to perform peak separation of diffraction lines from a {200} plane, a lattice constant calculated from a 200 diffraction angle was denoted by a, a lattice constant calculated from a 002 diffraction angle was denoted by c, and their ratio was determined as the average axial ratio c/a.

(b) In a case where the diffraction lines are not split clearly into two

A lattice constant calculated from a diffraction angle of a diffraction from a {200} plane was denoted by a, a lattice constant calculated from a diffraction angle from a {110} plane was denoted by c′, and their ratio c′/a was determined as the average axial ratio c/a.

Further, structure observation was performed under a SEM and a TEM to measure the average particle size of iron carbides present in a region having an area of 10 μm² or more, which was calculated as an equivalent circle diameter (dcar). Fine iron carbides that could not be identified with the TEM were measured by the atom probe method.

Subsequently, from the resultant steel sheets, tensile test specimens described in JIS Z 2241 (2011) were extracted with a direction perpendicular to a rolling direction (sheet width direction) taken as a longitudinal direction. Then, using the tensile test specimens, a tensile test was conducted in conformance with JIS Z 2241 (2011) to measure the mechanical properties (yield stress YS, tensile strength TS).

Further, in order to investigate collision resistances of the steel sheets, a collision test was conducted according to the following procedure, and the presence or absence of a fracture at that time was evaluated.

First, a steel sheet was subjected to bending or roll forming performed as a cold processing to be formed into a hat-shaped component A, and then the hat-shaped component A and a lid B were joined together by spot welding to be fabricated into a test piece having a shape illustrated in FIG. 1 . Next, the test piece was placed on a mount D such that A served as a top face, and a cylindrical weight C having a weight of 500 kg was caused to collide with a center portion of the test piece from a height of 3 m. Then, a region bent by the collision and an end face of the test piece were visually observed, by which evaluation of cracking was conducted. The evaluation was conducted according to a maximum length of cracks; the maximum length being 10 mm or more was rated as E, the maximum length being 7 mm or more to less than 10 mm was rated as D, the maximum length being 4 mm or more to less than 7 mm was rated as C, the maximum length being 2 mm or more to less than 4 mm was rated as B, and the maximum length being less than 2 mm was rated as A.

Results of the measurement and results of the evaluation are collectively shown in Tables 2 and 3. As is clear from the results shown in Tables 2 and 3, it is understood that example embodiments of the present invention, which satisfied all specifications, had yield stresses of 1000 MPa or more and caused no cracking after the collision test of their members. From the results, it is clear that the steel sheets according to the present invention are excellent in collision properties.

Example 2

Steels having compositions shown in Table 1 were melted and produced into slabs, and the slabs were heated at 1220 to 1260° C., subjected to rough rolling performed as a hot processing, subsequently subjected to finish rolling, and cooled to room temperature. Then, as shown in Table 4, the average cooling rate (CR1) for the range from the rolling finish temperature (FT) to 650° C. was changed, and for the range of 650° C. or less, cooling at the range from 10° C./s to 20° C./h was performed.

After the heat rolling was finished, the flattening was performed, and then the annealing was performed. In the annealing, the annealing temperature (ST), the annealing retention time (t1), and the average cooling rate (CR2) for the range from 700° C. to (Ms point - 50)° C. were changed, and in the heat treatment step, the holding time (t2) for the range from (Ms+50°) C to 250° C. was changed for steels having Ms being 250° C. or more, and the holding time (t3) for the range from (Ms+80°) C to 100° C. was changed for steels having Ms being less than 250° C. After the heat treatment step, skin-pass rolling for flattening was performed.

The resulting steel sheets were subjected to the measurement of the steel micro-structures and the mechanical properties and the evaluation of the collision resistance, as in Example 1. Results of the measurement and results of the evaluation are shown in Table 4.

TABLE 4 Aver- Eval- CR1 CR2 age uation Test FT (° C./ ST t1 (° C./ t2 t3 fM fA Bal- axial db dear YS TS of No. Steel (° C.) s) (° C.) (s) s) (s) (s) (%) (%) ance ratio (μm) (μm) (MPa) (MPa) cracking 63 A 900 10 830 30 60   400 99.8  0.2 1.0012 2.0 0.03 1120 1450 B Inventive example 64 B 900 10 830 30 60   400 99.8  0.2 1.0017 2.0 0.07 1120 1450 B Inventive example 65 C 900 10 830 30 60   400 92  2 B 1.0018 2.0 0.11 1260 1700 B Inventive example 66 D 900 10 840 30 60   500 90 10 1.0018 2.0 0.13 1040 1500 B Inventive example 67 900 10 800 30 60   500 83 11 F 1.0018 2.0 0.13  820 1480 B Comparative example 68 900 10 930 30 60   500 92  8 1.0118 3.5 0.12 1050 1490 E Comparative example 69 900 10 840  1 60   500 84 11 F 1.0004 2.0 0.16  830 1480 D Comparative example 70 900 10 840 30  5   500 82 11 B 1.0008 4.0 0.21  920 1480 E Comparative example 71 900 10 840 30 15   500 89 10 B 1.0008 2.4 0.19 1030 1490 B Inventive example 72 900 10 840 30  6     3 90 10 1.0110 1.8 0.003  960 1480 E Comparative example 73 900 10 840 30 60  1000 90 10 1.0015 1.8 0.13 1070 1490 B Inventive example 74 900 10 840 30 60 12000 90  6 B 1.0002 1.8 0.25 1120 1410 E Comparative example 75 900  5 840 30 60   500 90 10 1.0016 3.5 0.13 1070 1480 E Comparative example 76 H 900 10 840 30 60   900 96  4 1.0037 1.8 0.03 1310 1850 B Inventive example 77 I 900 10 800 30 60    600 96  3 B 1.0078 1.5 0.04 1450 2000 B Inventive example 78 900 10 720 30 60    600 83  5 F, B 1.0100 1.5 0.29  990 1950 B Comparative example 79 900 10 860 30 60    600 95  5 1.0098 3.4 0.03 1400 1980 E Comparative example 80 900 10 800  1 60    600 84  5 F 1.0028 1.6 0.16  980 1980 B Comparative example 81 900 10 800 30  5    600 82  5 B, F 1.0037 3.1 0.08  940 1970 E Comparative example 82 900 10 800 30 15    600 88  5 B 1.0037 1.6 0.17 1340 1990 B Inventive example 83 900 10 800 30 60      3 95  5 1.0240 1.6 0.001 1110 2020 E Comparative example 84 900 10 800 30 60     30 95  5 1.0220 1.6 0.003 1200 2010 E Comparative example 85 900 10 800 30 60    100 95  5 1.0090 1.6 0.016 1320 2010 B Inventive example 86 900 10 800 30 60   1000 95  5 1.0070 1.6 0.029 1410 2000 B Inventive example 87 900 10 800 30 60  12000 95  5 1.0019 1.6 0.028 1420 1980 B Inventive example 88 900 10 800 30 60 100000 95  5 1.0003 1.6 0.21 1530 1980 E Comparative example 89 J 900 10 820 30 60  400 99.8  0.2 1.0008 2.4 0.04 1030 1250 B Inventive example 90 K 900 10 830 30 60    600 93  7 1.0240 1.2 0.03 1500 2150 E Comparative example 91 L 900 10 830 30 60  400 83  0 F, B 1.0008 4.5 0.02  890 1150 E Comparative example

As is clear from the results shown in Table 4, it is understood that example embodiments of the present invention, which satisfied all specifications, had yield stresses of 1000 MPa or more and caused no cracking after the collision test of their members. From the results, it is clear that the steel sheets according to the present invention are excellent in collision properties.

Example 3

Steels having compositions shown in Table 1 were melted and produced into slabs, and the slabs were heated at 1220 to 1260° C., subjected to rough rolling performed as a hot processing, subsequently subjected to finish rolling, and the subsequent thermal history was changed. As shown in Table 5, the rolling finish temperature (FT) and the average cooling rate (CR3) for the range from the rolling finish temperature to (Ms -50°) C were changed. Further, in the heat treatment step, the holding time (t2) for the range from (Ms+50°) C to 250° C. was changed for steels having Ms being 250° C. or more, and the holding time (t3) for the range from (Ms+80°) C to 100° C. was changed for steels having Ms being less than 250° C. After the heat treatment step, skin-pass rolling for flattening was performed.

The resulting steel sheets were subjected to the measurement of the steel micro-structures and the mechanical properties and the evaluation of the collision resistance, as in Example 1. Results of the measurement and results of the evaluation are shown in Table 5.

TABLE 5 Test FT CR3 t2 t3 fM fA Bal- Average db dear YS TS Evaluation No. Steel (° C.) (° C./s) (s) (s) (%) (%) ance axial ratio (μm) (μm) (MPa) (MPa) of cracking  92 A 920 50  1000  99  1 1.0009 2.1 0.009 1130 1470 A Inventive example  93 650 50  1000  80  1 F 1.0010 2.3 0.01  810 1430 A Comparative example  94 920 20  1000  99  1 1.0010 2.2 0.01 1110 1470 A Inventive example  95 920  7  1000  82  1 B 1.0006 3.4 0.08  930 1330 E Comparative example  96 920  3  1000  45  1 B, F, F 1.0004 3.5 0.12  670  970 E Comparative example  97 920 50   200  99  1 1.0019 2.1 0.007 1100 1470 A Inventive example  98 920 50  5000  99  1 1.0006 2.1 0.04 1160 1470 D Inventive example  99 920 50  9000  99  1 1.0006 2.1 0.05 1170 1480 D Inventive example 100 920 50 18000  93  2 B 1.0002 2.1 0.21 1180 1470 E Comparative example 101 B 920 50  1000  99.8  0.2 1.0012 2.1 0.07 1100 1450 A Inventive example 102 C 920 50  1000  99  1 1.0018 2.0 0.11 1250 1700 A Inventive example 103 D 920 50  1000  90  4 B 1.0019 1.8 0.12 1060 1500 A Inventive example 104 920  7  1000  80  5 B 1.0013 3.1 0.12  950 1480 E Comparative example 105 920  3  1000  50 10 B, F 1.0004 3.3 0.18  820 1520 E Comparative example 106 920 50     2  99  1 1.0120 1.8 0.003 1050 1530 E Comparative example 107 920 50  1000  95  4 B 1.0030 2.0 0.005 1060 1530 A Inventive example 108 920 50  9000  90  9 B 1.0006 2.2 0.09 1020 1550 D Inventive example 109 920 50 18000  90  4 B 1.0002 2.2 0.21 1030 1560 E Comparative example 110 H 920 50  1000  97  3 1.0080 1.6 0.02 1280 1860 A Inventive example 111 I 920 50  97  3 1.0220 1.5 0.002 1390 1980 E Comparative example 112 920 50     3  97  3 1.0091 1.5 0.03 1470 1950 A Inventive example 113 920 50   800  90  8 B 1.0025 1.8 0.028 1260 1960 A Inventive example 114 920 50 18000  90  3 B 1.0002 1.7 0.21 1470 1920 E Comparative example 115 J 920 50  1000 60000 100 1.0007 2.2 0.012 1030 1250 A Inventive example 116 K 920 50  1000  95  5 1.0290 1.5 0.003 1510 2150 E Comparative example 117 L 920 50  1000 100 1.0009 4.5 0.13  960 1150 E Comparative example

As is clear from the results shown in Table 5, it is understood that example embodiments of the present invention, which satisfied all specifications, had yield stresses of 1000 MPa or more and caused no cracking after the collision test of their members. From the results, it is clear that the steel sheets according to the present invention are excellent in collision properties.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to obtain a high-strength steel sheet that exerts good reaction force properties when an impact load is applied to a shaped component from the steel sheet, is unlikely to cause a crack from an end face of the component or a region of the component bent at the time of the impact, and has a yield stress of 1000 MPa or more. The steel sheet according to the present invention is therefore suitable for a skeleton component and a reinforcing component of an automobile, and a component of building equipment or industrial equipment. 

1. A steel sheet having a chemical composition consisting of, in mass %: C: 0.14 to 0.60%, Si: more than 0% to less than 3.00%, Al: more than 0% to less than 3.00%, Mn: 5.00% or less, P: 0.030% or less, S: 0.0050% or less, N: 0.015% or less, B: 0 to 0.0050%, Ni: 0 to 5.00%, Cu: 0 to 5.00%, Cr: 0 to 5.00%, Mo: 0 to 1.00%, W: 0 to 1.00%, Ti: 0 to 0.20%, Zr: 0 to 0.20%, Hf: 0 to 0.20%, V: 0 to 0.20%, Nb: 0 to 0.20%, Ta: 0 to 0.20%, Sc: 0 to 0.20%, Y: 0 to 0.20%, Sn: 0 to 0.020%, As: 0 to 0.020%, Sb: 0 to 0.020%, Bi: 0 to 0.020%, Mg: 0 to 0.005%, Ca: 0 to 0.005%, and REM: 0 to 0.005%, with the balance: Fe and impurities, and satisfying following formulas (i) to (v), wherein a value of Ms expressed by a following formula (vi) is 200 or more, a steel micro-structure contains, in volume %: martensite: 85% or more, and retained austenite: 15% or less, with the balance: bainite, an average block size of martensite and bainite: 3.0 μm or less, an average axial ratio of martensite and bainite: 1.0004 to 1.0100, and a yield stress is 1000 MPa or more: Si+Al≤3.00  (i) C×Mn 0.80  (ii) Mn+Ni+Cu+1.3Cr+4(Mo+W)≥0.80  (iii) 0.003≤Ti+Zr+Hf+V+Nb+Ta+Sc+Y≤0.20  (iv) Sn+As+Sb+Bi≤0.020  (v) Ms=546 ×exp(−1.362 x C)−11 ×Si−30 ×Mn−18 ×Ni−20 ×Cu−12×Cr −8(Mo+W)  (vi) where symbols of elements represent contents (mass %) of the elements in the steel sheet, and in a case where an element is not contained, zero is assigned to its symbol.
 2. The steel sheet according to claim 1, wherein an average particle size of iron carbides included in the steel micro-structure is 0.005 to 0.20 μm.
 3. The steel sheet according to claim 1, wherein the steel sheet includes a plating layer on a surface of the steel sheet.
 4. A method for producing the steel sheet according to claim 1, wherein a cast piece having the chemical composition according to claim 1 is subjected to a hot-rolling step, a cold-rolling step, an annealing step, and a heat treatment step in this order, in the hot-rolling step, the steel sheet is cooled to room temperature at an average cooling rate for a range from a rolling finish temperature to 650° C. set at 8° C./s or more, in the annealing step, the steel sheet is held within a temperature range from an Ac₃ point to (Ac₃ point+100°) C for 3 to 90 s, and an average cooling rate for a range from 700° C. to (Ms point - 50°) C is set at 10° C./s or more, and in the heat treatment step, in a case where the Ms point is 250° C. or more, a holding time for a temperature range from (Ms point+50) to 250° C. is set at 100 to 10000 s, and in a case where the Ms point is less than 250° C., a holding time for a temperature range from (Ms point+80) to 100° C. is set at 100 to 50000 s, where the Ms point (° C.) and the Ac₃ point (° C.) are expressed by following formulas, where symbols of elements represent contents (mass %) of the elements in the steel sheet, and in a case where an element is not contained, zero is assigned to its symbol: Ms=546 ×exp(−1.362 ×C)−11 ×Si−30 ×Mn−18 ×Ni−20 ×Cu−12×Cr−8(Mo+W)  (vi) Ac₃₌910−203 ×C^(0.5)+44.7(Si+Al)−30 ×Mn+700 ×P−15.2 ×Ni−26 ×Cu−11 ×Cr+31.5 ×Mo  (vii).
 5. A method for producing the steel sheet according to claim 1, wherein a cast piece having the chemical composition according to claim 1 is subjected to a hot-rolling step, an annealing step, and a heat treatment step in this order, in the hot-rolling step, the steel sheet is cooled to room temperature at an average cooling rate for a range from a rolling finish temperature to 650° C. set at 8° C./s or more, in the annealing step, the steel sheet is held within a temperature range from an Ac₃ to (Ac₃₊₁₀₀)° C. for 3 to 90 s, and an average cooling rate for a range from 700° C. to (Ms−50°) C is set at 10° C./s or more, and in the heat treatment step, in a case where the Ms point is 250° C. or more, a holding time for a temperature range from (Ms+50) to 250° C. is set at 100 to 10000 s, and in a case where the Ms point is less than 250° C., a holding time for a temperature range from (Ms+80) to 100° C. is set at 100 to 50000 s, where the Ms point (° C.) and the Ac₃ point (° C.) are expressed by following formulas, where symbols of elements represent contents (mass %) of the elements in the steel sheet, and in a case where an element is not contained, zero is assigned to its symbol: Ms=546 ×exp(−1.362 ×C)−11 ×Si−30 ×Mn−18 ×Ni−20 ×Cu−12×Cr−8(Mo+W)  (vi) Ac₃₌910−203 ×C^(0.5)+44.7(Si+Al)−30 ×Mn+700 ×P−15.2 ×Ni−26 ×Cu−11 ×Cr+31.5 ×Mo  (vii).
 6. A method for producing the steel sheet according to claim 1, wherein a cast piece having the chemical composition according to claim 1 is subjected to a hot-rolling step and a heat treatment step in this order, in the hot-rolling step, a rolling finish temperature is set at a Ar₃ point or more, and an average cooling rate for a range from a rolling finish temperature to (Ms - 50°) C is set at 10° C./s or more, and in the heat treatment step, in a case where the Ms point is 250° C. or more, a holding time for a temperature range from (Ms+50) to 250° C. is set at 100 to 10000 s, and in a case where the Ms point is less than 250° C., a holding time for a temperature range from (Ms+80) to 100° C. is set at 100 to 50000 s, where the Ms point (° C.) and the Ar₃ point (° C.) are expressed by following formulas, where symbols of elements represent contents (mass %) of the elements in the steel sheet, and in a case where an element is not contained, zero is assigned to its symbol: Ms=546 ×exp(−1.362 ×C)−11 ×Si−30 ×Mn−18 ×Ni−20 ×Cu−12×Cr −8(Mo+W)  (vi) Ar₃₌910−310 ×C+33 ×Si−80 xMn−55 xNi−20 ×Cu−15 ×Cr−80 ×Mo  (viii).
 7. The steel sheet according to claim 2, wherein the steel sheet includes a plating layer on a surface of the steel sheet.
 8. A steel sheet having a chemical composition comprising, in mass %: C: 0.14 to 0.60%, Si: more than 0% to less than 3.00%, Al: more than 0% to less than 3.00%, Mn: 5.0 ⁰% or less, P: 0.030% or less, S: 0.0050% or less, N: 0.015% or less, B: 0 to 0.0050%, Ni: 0 to 5.00%, Cu: 0 to 5.00%, Cr: 0 to 5.00%, Mo: 0 to 1.00%, W: 0 to 1.00%, Ti: 0 to 0.20%, Zr: 0 to 0.20%, Hf: 0 to 0.20%, V: 0 to 0.20%, Nb: 0 to 0.20%, Ta: 0 to 0.20%, Sc: 0 to 0.2 ⁰%, Y: 0 to 0.20%, Sn: 0 to 0.020%, As: 0 to 0.020%, Sb: 0 to 0.020%, Bi: 0 to 0.020%, Mg: 0 to 0.005%, Ca: 0 to 0.005%, and REM: 0 to 0.005%, with the balance: Fe and impurities, and satisfying following formulas (i) to (v), wherein a value of Ms expressed by a following formula (vi) is 200 or more, a steel micro-structure contains, in volume %: martensite: 85% or more, and retained austenite: 15% or less, with the balance: bainite, an average block size of martensite and bainite: 3.0 μm or less, an average axial ratio of martensite and bainite: 1.0004 to 1.0100, and a yield stress is 1000 MPa or more: Si+Al≤3.00  (i) C×Mn 0.80  (ii) Mn+Ni+Cu+1.3Cr+4(Mo+W)≥0.80  (iii) 0.003≤Ti+Zr+Hf+V+Nb+Ta+Sc+Y≤0.20  (iv) Sn+As+Sb+Bi≤0.020  (v) Ms=546 ×exp(−1.362 x C)−11 ×Si−30 ×Mn−18 ×Ni−20 ×Cu−12×Cr −8(Mo+W)  (vi) where symbols of elements represent contents (mass %) of the elements in the steel sheet, and in a case where an element is not contained, zero is assigned to its symbol. 